Quorum-sensing blockade as a strategy for enhancing host defences against bacterial pathogens

Abstract

Conventional antibiotics target the growth and the basal life processes of bacteria leading to growth arrest and cell death. The selective force that is inherently linked to this mode of action eventually selects out antibiotic-resistant variants. The most obvious alternative to antibiotic-mediated killing or growth inhibition would be to attenuate the bacteria with respect to pathogenicity. The realization that Pseudomonas aeruginosa, and a number of other pathogens, controls much of their virulence arsenal by means of extracellular signal molecules in a process denoted quorum sensing (QS) gave rise to a new 'drug target rush'. Recently, QS has been shown to be involved in the development of tolerance to various antimicrobial treatments and immune modulation. The regulation of virulence via QS confers a strategic advantage over host defences. Consequently, a drug capable of blocking QS is likely to increase the susceptibility of the infecting organism to host defences and its clearance from the host. The use of QS signal blockers to attenuate bacterial pathogenicity, rather than bacterial growth, is therefore highly attractive, particularly with respect to the emergence of multi-antibiotic resistant bacteria.

Figure 1

The two main AHL signal molecules of P. aeruginosa, 3-oxo-C12-HSL and C4-HSL, the natural fungal QSI compounds, patulin and penicillic acid, and the synthetic halogenated furanones C-30 and C-56 and 4-nitropyridine-N-oxide.

Figure 2

QS-dependent tolerance of P. aeruginosa biofilms towards PMNs. Three-day-old biofilms of wild-type P. aeruginosa (a–c) and a ΔlasR rhlR mutant (d–g), both expressing GFP as a tag (grey), were exposed to PMNs for about 2 and 5 h. The PMNs appear black due to SYTO-62 staining. (b) Three-dimensional projection and (c) cross-section showing the wild-type biofilm with PMNs on top. (e) Three-dimensional projection and (f) cross-section showing the ΔlasR rhlR mutant fully penetrated by PMNs and the disappearance of much of the biomass. As seen from the enlargement of PMNs exposed to a QS-deficient biofilm frame (g) and isolated PMNs (h), the green fluorescent bacteria (grey) are attached to and phagocytosed by the PMNs. (Reproduced with permission from Bjarnsholt et al. (2005a).)

Figure 3

PMN activation measured by oxidative burst. Three-day old biofilms of wild-type P. aeruginosa and the ΔlasR rhlR mutant were exposed to PMNs for 2 h. The oxidative burst was visualized by the green fluorescence (light grey) emitted when 123-DHR is oxidized to 123-rhodamine due to production of H₂O₂. (a) Wild-type; (c) ΔlasR rhlR mutant. (b) Furanone C-30 and (d) Garlic extract mediated activation of PMNs present on a wild-type biofilm. The biofilm was grown for 3 days in the presence of 10 mM C-30 or 2% garlic extract. The PMNs fluoresce green (indicative of oxidative burst) compared with the PMNs in (a). (Reproduced with permission from Bjarnsholt et al. (2005a).)